So opto #4 is always closed in switch test?

Did you swap both transmitter and receiver opto?

Have you tested wire continuity for the receiver to ground and to the board connector?

With the balls in the trough during switch test you could easily jumper on of the other signals to opto 4 pin.
You should be able to jumper the 12v to that pin in switch test too but I’d need to look at the schematic to be safe.

I looked at the manual. Thinking out loud here...

Opto 1 a ball jam switch

If it is a ball jam, then when there are 4 balls in the trough, optos 2,3,4,5 should be closed. If 2,3,5 are closed and 4 is open, then it must be confused because they are out of order?

If 4 is always open and you have verified the emitters work, then the receiver is bad, the wiring is bad or something on the board. Have you jumpered the #4 opto with 12v at he board yet? It is J2-3 on the opto control board. Use J2-10 as the power to see if the switch closes in the test.

When you take a ball out does it give you a missing ball message? BEacuse based on the above, it would think it only has 2 balls.